Inversion or contrary motion is a counterpoint technique in which the interval direction is changed. Each ascending interval becomes a descending interval and vice versa:

The lower voice - due to augmentation - needs 8 measures to answer the first 4 measures. We have identified using lines and numbers each melodic idea presented by the upper voice. In these twelve bars the upper voice presents 3 melodic ideas of 4 bars each while the lower is only able to imitate the first. Between bars 5 and 12, Bach visits the D minor, G minor and B flat major keys. Here are measures 1 to 12:

This canon has two sections. In the first section (bars 1 to 52) the upper voice presents a total of 12 melodic ideas. The lower voice imitates only the first 6 ideas. On measure 53 the second section begins. This section is equal to the first 52 bars but with the voices inverted, that is, the voices exchange the melodic material. Now the upper voice is going to imitate the lower voice.

In bars 105 to 109 the top voice presents the subject for the last time and the canon concludes.